1. Field of the Invention
The present invention relates the fabrication of sandwich-type structural units from titanium materials, and more particularly to the provision of gas distribution channels in a layered stack of titanium alloy sheets, the channels enabling delivery of an expansion gas to the interior of the stack during superplastic formation of a sandwich structure.
2. Discussion of the Known Prior Art
In the family of titanium alloys, titanium aluminide materials have become well Known in the metallurgical arts as materials which exhibit excellent high-temperature strength and oxidation and creep resistance.
Titanium aluminides, like other titanium alloys, are metals which are relatively brittle and difficult to process and/or fabricate at or near room temperatures. One fabrication technique which has found widespread utility in the fashioning of structures for various industries is superplastic forming (SPF).
For many years, it has been Known that certain metals are "superplastic", i.e., have the capability of developing unusually high tensile elongations with reduced tendency toward necking. This property is exhibited by only a few metals and alloys and only within a limited temperature and strain rate range. Metals which appear to exhibit superplastic characteristics equal to or greater than those of any other metals are titanium, titanium alloys and, most recently, titanium aluminides. Indeed, with suitable titanium metals, it is possible to attain an overall increase in surface area of over 300%, and recent tests have shown these large elongations to be present in titanium aluminides as well.
The advantages of superplastic forming are numerous. Very complex shapes and deep drawn parts can be readily formed. Low deformation stresses are required to form the metal at the superplastic temperature range, thereby permitting forming of parts under low pressures which minimizes tool deformation and wear, allows the use of inexpensive tooling materials, and eliminates creep in the tool. Single male or female tools can be used; no spring-back occurs; no Bauschinger effect develops; multiple parts of different geometry can be made during a single operation; very small radii can be formed; and no problem with compression buckles or galling are encountered.
However, when carrying out the process of superplastic forming using titanium alloys, titanium aluminides and similar reactive metals, it is necessary to heat and form the materials in a controlled environment to ensure cleanliness of the titanium aluminide which is particularly sensitive to oxygen, nitrogen, and water vapor content in the air at elevated temperatures. Unless the titanium alloy material is protected, it becomes embrittled and its structural integrity is destroyed.
One solution to this problem has been to use pure argon gas as the uncontaminated environment in which to conduct superplastic forming of the titanium part. At the same time, the argon gas itself provides the mechanism for achieving the superplastic forming.
However, it has been found that with conventional forming apparatus, introduction and distribution of any gaseous medium for this purpose has been irregular and incomplete, thereby compromising the cleanliness of the forming environment and ultimately denigrating the structural integrity of the formed component.